The simplest form of protecting device is a spark gap arranged in parallel between earth and the high voltage terminal. With newer techniques the open spark gap device has been substituted by voltage arrestors made of unlinear material such as zinc oxide (ZnO). An example of such arrestors are described in ASEA data sheet LB 25.1 of 1. Dec. 1984, `ZnO--Ventilavledare typ XBE`. These known surge arrestors consist of a number of cylindrical ZnO blocks which are stacked one upon the other within an insulating cover. The ends of the stack are interconnected between earth and the high voltage terminal, with one end as close as possible to said terminal.